[lld][test] Fix darwin REQUIRES (NFC)

Some subprojects like compiler-rt define the `darwin` feature in their
lit config, but lld does not do that, so we need to use the global
system-darwin here instead. This test seems to have drifted from the
actual behavior so I also had to add `/usr/local/lib` here to make it
pass.

Differential Revision: https://reviews.llvm.org/D111268